1. Field of the Invention
The present invention relates to an image forming apparatus effectively producing quality color image without causing color slippage.
2. Discussion of the Background
A background color image forming apparatus generally includes a plurality of photosensitive drums as image carriers. It is known that there is a color slippage problem on a formed image due to a deviation of rotational speed shifts at each photosensitive drum. Each photosensitive drum is generally unified with a drive transfer member engaging with each other photosensitive drum and is driven by a driving force via the drive transfer members.
The deviation of rotational speed shifts is caused by an accuracy error and an eccentricity generated at an attachment process of the drive transfer member. Although this problem has been studied, no clear solution to solving this problem has yet been obtained. It has been proposed to adjust each phase of a rotational speed shift to a phase of a rotational speed shift of a drive transfer member so that an amount of color slippage is minimized by restraining the deterioration of image quality.
More specifically, a background method for adjusting the rotational speed shift is to adjust a phase of the each rotational speed shift within a predetermined minimum range. And the method includes the following procedure. At first, color slippage adjusting patterns are formed on intermediate carriers such as intermediate transfer belts and then are read by pattern detection apparatuses. The rotational speed shift and the amount of phase shift on each photosensitive drum axis are calculated.
Further, phase differences between the photosensitive drums are calculated until getting a result for achieving a minimum amount of color slippage. The result of the phase information is stored as profile data in a storage device, such as a rewritable nonvolatile memory. Then, phase relationships of the photosensitive drums are adjusted to follow the profile data calculated based on the phase information before an image forming process.
In the background image forming system, a base mark (marking) which shows a rotating position is arranged at each drive gear and a dedicated member of a photosensitive drum to adjust to follow the profile data. Marking detection sensors are arranged firmly at a main unit of the apparatus to detect the marking.
The markings of the photosensitive drums are detected by the marking detection sensors at every rotation. Then, differences of detection timing of the markings are determined. Based on the difference, each drive motor for driving each photosensitive drum is controlled to align the rotation phase to a phase of a photosensitive drum.
Namely, in this configuration, a detection signal is transmitted when a marking is passing through a detection zone during a rotation of each photosensitive drum. Then, the rotation phase is calculated from the timing differences of the transmitted signals. Then, each rotation phase is aligned by controlling the drive motor for each photosensitive drum in a predetermined manner to the profile data previously determined.
In this background configuration, the phase relationships between photosensitive drums are determined using the signals detected while each marking is passing through the marking detection sensor in accordance with the rotation of each photosensitive drum. For this reason, it may take a relatively long time to print a first page.
For example, when a position of the marking on the drive transfer member is in a position where it is just passing over the marking detection sensor at a start timing of a phase detection procedure, it is needed to wait until the marking rotates almost one more turn and passes through the marking detection sensor. This causes a time lag for detection and makes a delay for the first page printing.
To solve this time lag problem, it has been proposed to make a marking larger and to place a plurality of the markings, to detect the markings more efficiently and determine the rotation position in a shorter time. More specifically, it has been proposed to arrange a longer marking in a rotating direction and a plurality of markings that have different lengths on a circumference.